This invention pertains to semiconductor technology, and more particularly to storage cell capacitors for use in dynamic random access memories.
As memory devices become more dense it is necessary to decrease the size of circuit components. One way to retain the storage capacity of a dynamic random access memory (DRAM) device and decrease its size is to increase the dielectric constant of the dielectric layer of the storage cell capacitor. In order to achieve the charge storage efficiency needed in 256 megabit(Mb) memories and above, materials having a high dielectric constant, typically greater than 50, can be used as the dielectric layer between the storage node and the cell plate capacitor electrodes. The dielectric constant is a value characteristic of a material and is proportional to the amount of charge that can be stored in the material when it is interposed between two electrodes. BaxSr(1xe2x88x92x)TiO3 [BST], BaTiO3, SrTiO3, PbTiO3, Pb(Zr,Ti)O3 [PZT], (Pb,La)(Zr,Ti)O3 [PLZT], (Pb,La)TiO3 [PLT], KNO3, and LiNbO3 are among some of the high dielectric constant materials that can be used in this application. These materials have dielectric constant values above 50 and will likely replace the standard Si3N4, SiO2/Si3N4, Si3N4/SiO2, or SiO2/Si3N4/SiO2 composite films used in 256 kilobits (Kb) to 64 megabits (Mb) generations of DRAMs. Si3N4 and SiO2/Si3N4 composite films have dielectric constant values of 7 or less. The storage node and cell plate electrodes are also referred to as first and second electrodes.
Unfortunately high dielectric constant materials are incompatible with existing processes and can not be simply deposited on a polysilicon electrode as was the case for the lower dielectric constant materials, such as Si3N4 and SiO2/Si3N4 composite layers. This incompatibility is a result of the O2 rich ambient present during the high dielectric constant material deposition or during annealing steps. The O2 oxidizes portions of the materials formerly used for the storage node plate. Also the capacitors employing the former materials undergo physical degradation during thermal cycles due to the diffusion of the storage node plate material into the dielectric.
In the storage cell capacitor incorporating BST, described in the IDEM-91 article entitled, A STACKED CAPACITOR WITH (BaxSr1xe2x88x92x)TiO3 FOR 256M DRAM by Koyama et al., some of these aforementioned problems are resolved. The storage node electrode typically comprises a layer of platinum overlying a tantalum barrier layer which, in turn, overlies a polysilicon plug. Platinum is used as the upper portion of the first electrode since it will not oxidize during a BST deposition or subsequent anneal. An electrode that oxidizes would have a low dielectric constant film below the BST, thereby negating the advantages provided by. the high dielectric constant material. The tantalum layer is introduced to avoid Si and Pt inter-diffusion and to prevent the formation of SiO2 on top of the platinum surface. In addition, the platinum protects the top surface of the tantalum from strong oxidizing conditions during the BST deposition. FIG. 1 depicts the stacked storage node electrode of the related art comprising tantalum 1, platinum 2 overlying the polysilicon plug 3.
The sidewalls 4 of the tantalum film 1 formed during this process are subject to oxidation during the subsequent deposition of the BST layer. Since the tantalum 1 oxidizes, the polysilicon plug 3 is also susceptible to oxidation. When portions of the polysilicon plug 3 and tantalum 1 are consumed by oxidation the capacitance of the storage cell capacitor is decreased since the storage node electrode is partially covered by a low dielectric constant film. Therefore the memory device cannot be made as dense.
In addition, the storage node contact resistance increases drastically between the polysilicon plug and the tantalum barrier layer as a result of degradation of the tantalum barrier layer during deposition of BST and other high dielectric constant materials.
An object of the invention is to increase density of a memory device by increasing capacitance of storage cell capacitors.
A further object of the invention is decreased contact resistance between the polysilicon electrode and the barrier layer and reduced degradation of the barrier layer.
The storage cell capacitor of the invention features a storage node electrode having a barrier layer which prohibits the diffusion of atoms. The barrier layer may be titanium nitride or another material which prohibits silicon diffusion. The barrier layer is interposed between a conductive plug and a non-oxidizing conductive material, typically platinum. A dielectric layer, typically BaxSr(1xe2x88x92x)TiO3 [BST], is deposited on the non-oxidizing material. The barrier layer is surrounded on its sides and exposed upper portions by one or more insulative layers.
The insulative layers and the non-oxidizing material protect the barrier layer from oxidizing during the deposition and anneal of the BST thereby also eliminating oxidation of the conductive plug. By eliminating oxidation of the barrier layer and oxidation of the conductive plug, capacitance is maximized, and the contact resistance is not affected.
Optionally, the invention also features a low contact resistance material lying between the conductive plug and the barrier layer.
The invention is a storage node capacitor and a method for forming the storage node capacitor having a storage node electrode comprising a barrier layer interposed between a conductive plug and an oxidation resistant conductive layer. The method comprises forming the conductive plug in a thick first layer of insulative material such as oxide or oxide/nitride. The conductive plug is recessed from a planarized top surface of the first insulative layer. The optional low contact resistance material is formed at the base of the recess. The barrier layer is then formed in the recess.
Next a second layer of insulative material is formed overlying the first layer of insulative material and optionally overlying a portion of the barrier layer. The process is continued with a formation of an oxidation resistant conductive layer overlying the barrier layer not covered by the second insulative layer.
Next a dielectric layer, typically having a high dielectric constant, is formed to overly the oxidation resistant conductive layer and a further conductive layer is fabricated to overly the dielectric layer.
Since the barrier layer is protected during the formation of the dielectric layer by both the oxidation resistant conductive layer and the thick insulative layer there is no oxidation of the barrier layer or the contact plug, thereby maximizing capacitance of the storage node and reducing high contact resistance issues.